Vented or Unvented Cylinder

The difference between a vented and an unvented cylinder is where the pressure comes from. A vented cylinder is fed by a cold tank in the loft, and the pressure at your taps is created by the height of that tank above them, which is why the shower on the top floor is always the weakest in the house. An unvented cylinder is sealed and fed straight from the mains, so every outlet gets mains pressure and there is no tank at all.

Which one suits you comes down to the water arriving at the property, the space you have, and what you want the showers to do. Only one of them is notifiable work, because an unvented cylinder stores hot water under pressure and has to be installed by somebody holding the right qualification for it. We are a Gas Safe registered and OFTEC certified business based in Bearsted, and what follows is how the two genuinely compare, including the one check that settles it more often than anything else.

How each one gets its pressure

A vented system is gravity fed. Cold water sits in a tank in the loft, drops down into the cylinder, and the further it falls before it reaches an outlet the more pressure it arrives with. That is the whole mechanism, and it explains every quirk of a vented system. Downstairs taps are strong, the top floor is weak, and an en suite in a loft conversion barely works at all because there is almost no height left above it.

An unvented cylinder has no tank. Mains water goes into it under mains pressure and comes out under mains pressure, so a shower on the second floor performs much the same as one on the ground floor. It also frees up the loft, which is often the real reason people ask about it in the first place.

Direct or indirect is a separate question

There is a second distinction worth getting straight, because it is not the same question and the two get muddled constantly. A direct cylinder is heated only by immersion heaters, on electricity. An indirect cylinder is heated by your boiler through a coil inside it, usually with an immersion heater kept as a backup.

Almost any household with a working boiler wants indirect, because heating a full cylinder on electricity alone is an expensive way to run a bath. Vented and unvented both come in direct and indirect versions, so the two decisions are made separately and a quote should say which of each you are getting.

The check that actually decides it

An unvented cylinder does not create pressure, it passes on whatever arrives. So the thing that decides whether one is worth fitting is not the number of bathrooms, it is the flow rate of the incoming main, and that is measured rather than guessed. If the supply into the house is a narrow old pipe, or shared with a neighbour, an unvented cylinder will faithfully deliver mains pressure that was never worth having. We have found this is far and away the most common reason an unvented upgrade disappoints, and it is entirely predictable beforehand with a flow test that takes a few minutes.

It is also the question a lot of quotes skip, because measuring it means turning up before pricing the job. If somebody has quoted you for an unvented cylinder without testing what the main can do, that quote is a guess with a number on it.

Why an unvented cylinder is regulated

A vented cylinder is open to the atmosphere through its vent pipe, so it cannot build pressure. An unvented one is sealed, and water expands as it heats. That expansion has to go somewhere, and if every route is blocked the stored energy in a cylinder of very hot water becomes genuinely dangerous. That is the entire reason for the safety kit and the paperwork.

Installing one is notifiable under Part G of the building regulations, which is where the requirements for unvented hot water systems are set out. The work has to be carried out by somebody qualified for unvented systems and notified to building control, and the cylinder needs an annual check to keep the safety components working and usually to keep the warranty valid too.

Where an unvented one can actually go

An unvented cylinder brings a siting constraint that people rarely see coming. All of that safety kit has to discharge somewhere, and the discharge pipe needs to reach a point outside where very hot water can escape safely and be seen doing it. It has to fall continuously along its whole run, it cannot go uphill, and there are limits on how far it can travel and how many bends it can take on the way.

In practice that rules out a good number of the cupboards people have in mind. The classic one is an airing cupboard in the middle of an upstairs landing with no external wall anywhere near it. The cylinder itself would fit perfectly well, and the discharge has nowhere legitimate to go.

From working with customers across Kent, this is the detail most likely to change a plan late in the day, which is why we look at it first rather than last. If somebody has quoted for an unvented cylinder without establishing where the tundish discharges to, that question has not gone away. It will simply surface later, at somebody's expense.

What goes wrong with each

They fail in different ways, and knowing which you have tells you what to expect.

• On a vented system, the loft tank is the weak point. It is open, so it collects dust and worse over the years, and in an unheated loft it is the first thing to freeze. The cylinder itself is usually copper and will often outlast everything around it.
• On an unvented system, the expansion vessel loses its charge over time. The symptom is water dripping from the tundish or the discharge pipe outside, which people often assume is a leak when it is a safety valve doing exactly what it should.
• On both, the immersion heater and the coil scale up, and in a hard water area that is a matter of when rather than whether.
• On both, a cylinder thermostat can drift, so the water is either never hot enough or is being stored hotter than it needs to be, which costs money quietly.

That third one matters more in Kent than it does in much of the country. Hard water furs up the coil on the outside and the immersion element on the inside, and a cylinder that holds water hot for hours is exactly the condition scale is looking for. It is the main reason a cylinder here does not last as long as the same one would in Manchester.

Which one we would recommend

If the mains flow is good and the loft is spoken for, unvented is usually the better answer and it is what most people want once they have felt the difference at the shower. If the mains flow is poor, a vented system with a decent pump can be the more honest recommendation, and it is certainly the cheaper one.

In our experience, an unvented cylinder works better than a vented one in a house where the loft has been converted, because a vented system depends on the height of a tank that no longer has anywhere to go, and pumping a gravity system rarely matches what the mains will do on its own.

Size matters as much as type, and it is the thing people underspecify. A cylinder is a battery, and once it is flat you wait for it to recharge. Two bathrooms in regular use want more storage than one, and a household that showers in the same half hour every morning wants more than one that spreads out. We would rather talk that through properly on a cylinder replacement than fit like for like and have somebody run out of hot water in a year's time.

So, vented takes its pressure from height, unvented takes it from the main, and the flow rate at your stop tap decides whether the second one is worth doing. Everything else follows from that. We cover Maidstone and the rest of Kent from Bearsted, with free home visits and free quotes across Kent. If you are weighing the two up, get in touch and we will test the main before we tell you what to buy.